I have an HP ProBook 6450b laptop which runs Ubuntu 12.10 and Windows 7. It had a tedious and potentially expensive problem: when running Linux, the CPU fan would fail to turn on until the CPU temperature reached 105℃. This was dangerously hot. I wrote about the problem in detail on the Ubuntu Forum, including links to my conversations on the linux-acpi kernel list.

After spending about a month trying to find a software fix for this problem, I decided it was time to get the soldering iron out to fix the problem. Here’s what I did:

  • I followed the official HP guide for removing the keyboard (it’s really easy).
  • There are three wires running from the motherboard to the CPU fan:
  • I cut the blue wire and left it disconnected.
  • I cut the red wire and tinkered with different resistors spliced into the red wire. I searched for a fan speed which would be quiet but produce significant cooling:
  • I settled for two resistors in parallel:
  • One resister was a 47Ω (yellow, purple, black) the other was a 470Ω (yellow, purple, brown) giving a final resistance of 42.7Ω.
  • The finished product (not very tidy but it works!).

(disclaimer: I’m not even vaguely qualified to tinker with laptop hardware. I’m just some dude from the internets. I hacked together a solution which worked for me and I’ve done my best to report what I did in the hopes that this solution might work for others. If you break your laptop trying to follow these steps then I’m sorry but I can’t be held responsible.)